Stator windings in electric machines are typically wound in the stator by looping wire through the slots of a stator assembly. Although low in cost, this method of making motors and generators is limited to moderate power machines. For higher power machines, preformed coils that fit into the slots of the stator assembly are preferred. Each coil has a substantially rectangular shape that closely matches the shape of a slot. The geometry of the motor favors the use of identical coils for all of the phases.
Recently, progress has been made in the design of slotless DC machines. Slotless machines are sometimes used in high speed motors for which the magnetic losses associated with the teeth in slotted machines are prohibitive. Low cost motors often utilize slotless configurations because of the high cost of inserting windings between teeth. Slotless machines are also used in precision applications to avoid other problems that occur with teeth. For example, cogging torque caused by the teeth is responsible for non-uniform angular velocity and torque ripple, and is particularly problematic in low power motors.
Advances in superconductors and permanent magnet materials now make it feasible to develop high power slotless machines. For example, coil configurations have been developed for axial gap, radial gap and linear slotless machines. Similar to existing slotted coil designs, these slotless coil configurations typically include coil thicknesses that match or exceed the coil width. Unfortunately, coil configurations for slotless machines generally do not satisfy the size, cost and performance requirements necessary to produce a commercially practical slotless machine utilizing radial magnetic fields. The coils have complex bends that increase fabrication costs. Moreover, the thickness of the coils decreases the magnetic efficiency of the machine. In addition, the radial build, or increase in the effective radius of the stator core, due to the end turns of the coils increases the overall motor size.
Accordingly, there exists a need for a coil configuration that is wide and thin, and includes short end turns with minimal radial build. The coils should be easily fabricated using standard materials for low cost. The present invention satisfies these needs and provides additional advantages.